1. Field of the Invention
The present invention generally relates to comparators. More specifically, the present invention is directed to reducing an offset of a comparator using an auto-zeroing technique.
2. Related Art
A comparator is designed to compare an input signal to a known reference level. The input signal can be an input voltage or an input current. Correspondingly, the known reference level can be a voltage reference level or a current reference level. The ideal reference level of the comparator is exactly zero. Typically, the comparator is designed to output a logic “1” at the end of a clock cycle, when an input signal exceeds the known reference level, and to output a logic “0” at the end of the clock cycle, when the input signal is below the known reference level.
Transistors arranged to provide positive feedback are typically used to implement the comparator. The reference level of the comparator is influenced by the threshold voltages of the transistors. Mismatches in the physical characteristics of the transistors used to implement the comparator cause the threshold voltages of the transistors to differ. Consequently, a difference between the threshold voltages of the transistors often causes the reference level of the comparator to deviate from the ideal level. The amount of deviation from the ideal reference level is represented as either a voltage offset or a current offset. This comparator offset can cause the comparator to provide an incorrect output value for a given input value, resulting in comparator inaccuracy. As the input signal becomes smaller, the comparator becomes increasingly prone to inaccuracies caused by the comparator offset.
Comparators are basic building blocks of an Analog-to-Digital Converter (ADC). Comparator offset is an important parameter of the ADC. A common technique used to minimize the effect of the comparator offset involves preamplifying the input signal of the comparator. In many ADC architectures, such as flash ADCs, folding ADCs or two-step flash ADCs, the amount of comparator offset determines how much preamplification is needed on the front-end of a comparator array.
An alternative to preamplification of the input signal is offset compensation. In ADCs where many comparators operate in parallel, improving the offset performance of the comparators can lead to substantial area and power savings in the ADC design. That is, comparators that have lower offset resulting from offset compensation require less preamplification, and therefore fewer stages of preamplifiers, in their front-ends.
Auto-zeroing techniques are often used to reduce the offset of the comparator. During a reset phase of the comparator, the offset of the comparator is stored on capacitors. During the succeeding latch phase, the comparator produces a comparator output after transitioning from a meta-stable state to a stable state. The offset stored during the reset phase of the comparator is used to compensate for the offset of the comparator during the latch phase. This compensation technique helps minimize the effect of the comparator offset during the latch phase.
Many auto-zeroing techniques have been developed. However, these techniques typically require a significant amount of additional circuit components. The use of additional circuit elements increases the area, and often the power consumption, of the comparator. For example, several auto-zeroing techniques require a large number of extra capacitors to accomplish offset compensation. The introduction of a large number of capacitors, in turn, requires considerable chip area. Other auto-zeroing techniques require the addition of an amplifier to perform offset compensation, which increases power and space requirements. Lastly, techniques which use the source-gate capacitances of additional transistors increase the power consumption of the comparator due to the biasing requirements of the introduced transistors. Further, these techniques require the introduction of a large number of additional switches.